Rechargeable energy storage cells are used in a variety of applications including gas operated motor vehicles and electric vehicles. Of the applications, electric vehicles such as golf carts, fork trucks, marine applications, floor-sweeper scrubber and recreational vehicles and the like are the most demanding in terms of charge/discharge cycles. Energy storage cells for such electric vehicles are commonly known as “deep cycle” batteries which provide relatively steady power over extended periods of time between charging and which operate in a deep cycling mode of severe discharging as well as daily recharging cycles. Deep cycle energy cells are desirably recharged with little or no supervision. Accordingly, the cells must be capable of multiple charge/discharge cycles without significantly degrading of the power output properties of the cells. Conventional rechargeable energy storage cells, such as automobile and golf cart batteries, have properties which do not lend themselves to the rigorous duty cycles of the deep cycle batteries.
Most energy storage cells of the nature described above contain positive plates or grids made of lead-antimony alloys which are easier to cast and produce stronger electrodes. The antimony also improves the corrosion resistance of the positive plate to acid attack and increases the ability of the battery to recovery from deep discharge cycles. When a lead-acid energy storage cell is deeply discharged the energy storage cell must be recharged immediately or the battery life is compromised. The need for immediate recharging relates to the fact that when a lead-acid energy storage cell is discharged the acid is temporarily consumed and the electrolyte becomes more like water. Under these conditions the active material in the plates may dissolve, and diffuse into the separator, filling the pores with lead ions. Once the pores become filled with lead ions, the effectiveness of the separator is dramatically reduced and electrical shorting of the energy storage cell may occur in a short period of time.
There are several types of separators which are commercially used in rechargeable energy cells. The separators differ by the material composition and include rubber separators, polymeric separators such as polyethylene separators, polyvinyl chloride (PVC) separators, phenolic resorcinol separators, fiberglass separators and resin impregnated cellulosic paper separators. The separators are further classified as microporous separators and macroporous separators.
The microporous separators include separators made of natural rubber, polyethylene, phenolic resin, PVC and polymeric membranes. Macroporous separators include separators made of glass fiber mats, sintered PVC and resin-impregnated cellulosic papers. Of the foregoing, microporous, natural rubber separators typically exhibit the best electrochemical performance characteristics which enhance the overall performance of the energy cell.
There are two primary functional aspects of separators used for energy cells, one is physical and the other electrochemical. The important physical characteristics include high porosity, small mean pore diameter, oxidation resistance, puncture resistance, thermal dimensional stability and low levels of harmful chemical contaminants. Electrochemical characteristics of importance include favorable voltage characteristics, retardation of antimony transfer, acceptable Tafel behavior, and prevention of dendrite growth. The Tafel behavior of an energy storage cell is a determination of the hydrogen and oxygen over-potential shifts in the cell electrolyte compared to pure acid solutions.
Valve regulated lead-acid energy storage cells typically use separators made of 100% non-woven glass fibers that facilitate oxygen recombination. Under certain conditions, the oxygen recombination may become too vigorous causing the energy storage cell to go into thermal runaway. If the charger used for charging the energy storage cell is not temperature compensated, the energy storage cell may eventually melt and, in severe cases, ignite or burn.
Despite the advances made in the art with respect to improved separators, there continues to be a need for separators for energy storage cells which exhibit improved physical and electrochemical properties over conventional separators. For example, in automotive applications there continues to be a need to improve the cold cranking performance of the energy storage cells. Improvements are also desirable for applications such as golf carts which may benefit from increasing the deep cycle life of the energy storage cells designed for such cycling applications. Primary electrochemical energy storage cells may also benefit from improved low temperature properties.
With regard to the above, one embodiment of the disclosure provides a separator for an energy storage cell having a microporous matrix including a reversible porosity-controlling agent. The reversible porosity-controlling agent is selected from the group consisting of agents that change size as a function of temperature, agents that change size as a function of electrolyte concentration, and agents that change size as a function of temperature and electrolyte concentration to provide a change in an overall porosity of the separator.
Another embodiment provides a method for improving an operating characteristic of an energy storage cell. The method includes applying from about 0.05 to about 20 weight percent of a reversible porosity controlling agent to a separator material. An improved separator may be formed from the separator materials and a reversible porosity-controlling agent. The porosity-controlling agent may be selected from agents that change size as a function of temperature, agents that change size as a function of electrolyte concentration, and agents that change size as a function of temperature and electrolyte concentration. The energy storage cell is then operated with the separator.
The separators according to the invention exhibit improved properties as compared to conventional separators. Another advantage of the disclosed embodiments is that the separators may take an active rather than passive role in improving the performance of energy storage cells under variable conditions. Until now, energy storage cell separators have been a passive component of the cells, with the exception of the tri-layer thermal shutdown separator used in the lithium-ion battery industry (see U.S. Pat. No. 5,952,120 and others assigned to Celgard). The “shutdown” separator is a three-layer structure of stretched polypropylene/poly-ethylene/polypropylene. The internal layer of PE is designed to melt at high temperatures thus increasing the electrical resistance of the storage cell and “shutting down” the energy storage cell. The process of “shutting down” the energy storage cell is irreversible and once this occurs the energy storage cell is non-functional and must be replaced. By comparison, the separators described herein may be used to shut down an energy storage cell under high temperature conditions, and may enable the storage cell to be used again when conditions return to normal. In other words, the separators have substantially reversible properties that enable the energy storage cells to continue to be used, yet provide protection during deep discharge cycles.
Another advantage of the separators and energy storage cells containing the separators as described herein is that the separators may include components that increase the porosity of the separator under relatively low temperature conditions so that increased power from the energy storage cell may be obtained.